This invention relates to a booster device employed, for instance, for a brake system in a motor vehicle, and more particularly to the structure in a booster device of this type which includes a key member to prevent a valve plunger from coming off from a valve body (hereinafter referred to as "a valve plunger retaining structure", when applicable).
In general, a booster device comprises: a power piston which is slidably arranged in a shell; a constant pressure chamber formed on the front side of the power piston, and a variable pressure chamber formed on the rear side of the power piston; a valve body provided in the central axial region of the power piston; a valve mechanism provided in the valve body; a variable pressure passage formed in the valve body through which the valve mechanism is communicated with the variable pressure chamber; an input shaft which operates in association with a valve plunger forming the valve mechanism, to switch flow paths in the valve mechanism; and a key member engaged with the valve plunger, to prevent the valve plunger from coming off the valve body.
The following structure for the booster device is also known in the art: In general, the variable pressure passage includes an axial passage which is extended from the valve mechanism along the axis of the valve body towards the front end of the booster device, and a radial passage which is extended radially from the front ends of the axial passage, and the key member is inserted into the radial passage to engage with the valve plunger. In this case, the radial passage is used as a hole into which the key member is inserted.
With the booster device, it is necessary to design the key member as follows: That is, both end faces of the key member are abutted against the valve body, so that the key member may not be axially moved; or the key member is axially movable a certain distance. However, in the case where the radial passage is used as the key-member inserting hole, the aforementioned axial passage is formed between the outer cylindrical surface of the rear part of the valve plunger and the inner cylindrical surface of the valve body, and the axial passage is communicated with the radial passage in which the key member is inserted. Hence, the abutment of the valve body against the rear end face of the key member occurs radially outside the axial passage.
On the other hand, the valve plunger is provided along the central axis of the axial passage in such a manner that it is axially movable. Hence, the region where the key member engages with the valve plunger is located on the axis central portion of the axial passage (hereinafter referred to as "an engaging region", when applicable). Therefore, the distance between the engaging region and the region where the key member abuts against the valve body (hereinafter referred to as "an abutting region", when applicable), is unavoidably relatively large. Hence, if, when the brake pedal is released to retract the valve plunger, the latter collides with the key member, a great bending moment is applied to the key member, so that the latter may be bent with high probability. This difficulty may be overcome by increasing the rigidity of the key member. However, the method gives rise to another difficulty that the key member is increased in thickness and accordingly in weight.